A picture and motherboard brand & model would help. Is it a EATX motherboard?If you can't fit the fourth card physically because the graphic cards are double-width, there's not much you can do if you want something integrated with your air cooling system.Unless it could fit with some cuttings in the back panel or something, or replace the motherboard or computer case with one that supports 4 cards.You can also replace the single cards with dual ones, like the Radeon 6990 (that's out of stock.)Check hardware or overclockers forums such as hardocp or the hardware section in this forum.